Known methods adjust a supplemental quantity of fuel during warm-up which is comparatively large at the start and is reduced in accordance with a predetermined function with increasing duration of the warm-up and therefore with increasing warming of the engine. At the latest, no supplemental quantity of fuel is supplied any more when the operating temperature of the engine is reached. U.S. Pat. No. 4,205,635 discloses an example of this known method.
The necessity of a warm-up enrichment by increasing the quantity of fuel is caused by the so-called wall-film losses. These losses occur because a portion of the fuel injected into the intake pipe of the engine condenses on the inner wall surfaces of the intake pipe and forms a fuel wall film. This portion of the injected fuel is not available for combustion in the cylinder. Its magnitude is also determined by the condensation behavior and boiling behavior of the fuel used. The fuel quantity injected during warm-up is increased in order to obtain a desired fuel/air ratio .lambda. notwithstanding the wall-film effect.
In accordance with the known method, the warm-up enrichment takes place in a controlled manner such that a specific fuel quality leads to a desired .lambda. in the cylinder. A comparatively lean warm-up adaptation is advantageous for present day exhaust-gas strategies. For the comparatively lean warm-up adaptation, driving problems can occur because of a mixture which is too lean because of seasonal and regionally fluctuating qualities of fuel or even because of changes of the through-flow characteristic of the injection valve caused by deterioration. A conventional lambda control cannot eliminate these problems because it is not active during warm-up for various reasons.